Optical communication is in widespread use, such as in telephonic and data networks. Fiber optic networks efficiently and reliably handle massive amounts of voice and data communications. The benefits of optical communications compared to wired communications are well known, and include higher speed, larger bandwidth and reduced vulnerability to interference.
These known advantages have created significant interest in micro and nanoscale optical devices and systems. Several applications for such devices and systems provide the impetus for research and development of new optical amplifiers. Over the past two decades, considerable effort has, therefore, been directed toward developing such nanoscale and microscale optical amplifiers but those available suffer from one or more drawbacks. For example, conventional micro/nano optical amplifiers include semiconductor based devices that require sophisticated and expensive fabrication systems such as MOCVD or MBE reactors, and focused ion beam and plasma etching tools. Furthermore, semiconductor optical amplifiers are restricted to specific spectral regions that are dictated by the electronic structure of the material. Other microscale and nanoscale optical amplifiers often produce a host of emission (spectral) lines and present impractical integration issues.
Most of the amplifiers and optical lasers (oscillators) that have been demonstrated to date are based on the III-V or II-VI semiconductors. These devices generally have thin layers of semiconductor materials that are bounded by mirrors (often a stack of thin films) or are encapsulated by metal. One drawback of these devices is the inherently complex and expensive fabrication processes, and equipment, for formation of one or more optical emitters, and the requirement that the layers forming the gain medium must be crystalline. Another drawback is that the semiconductors are inherently limited to specific regions of the visible, ultraviolet, and near-infrared.
Other types of microlasers and amplifiers have been reported in which a gain medium is provided in conjunction with a whispering gallery mode (WGM) resonator. The resonator can be shaped as a sphere, ring, or toroid. For example, a group at the California Institute of Technology has obtained lasing in glass microspheres made from Er-doped glass. The first lasers of this type were a calcium difluoride crystalline sphere (in the early 1960s) and, subsequently, droplets of a solvent into which laser dye was dissolved. Such lasers do not require a conventional optical resonator because the optical mode circulates within the resonator, around its periphery. A gain medium combined with a solvent is impractical for many applications. Also, coupling power out of a resonator medium often requires a tangential waveguide or a tapered fiber, and is difficult to accomplish reliably. Another concern is that the output spectrum is often multi-line. This is a serious drawback for on-chip communications, computing, or sensing applications.
Whispering gallery mode resonators have been studied in the microscale. Pump thresholds of only a few photons per whispering gallery mode have been observed, and Raman gain coefficients for a nonlinear whispering gallery mode resonator are increased by two orders of magnitude relative to bulk values. See, Lin, H. & Campillo, A. Microcavity enhanced Raman gain. Opt. Commun. 133, 287-292 (1997). Ahn et al. employed plasmonic nanoantennas to deliver optical radiation, by free-space transmission, to a spherical resonator with a coupling efficiency of 44%. Aligned with the equatorial plane of the microsphere, the nanoantennas were separated from the sphere surface by a mean distance of ˜100-150 nm and radiated into the evanescent optical field of the resonator. Toroidal resonators have also been demonstrated by the Vahala Research Group at the California Institute of Technology.
Plasmonic structures have also been considered and demonstrated to act as nanoscale and microscale amplifiers and lasers. These structures have been shown to be effective as nanoantennas for both lasers and fluorescent optical sources. See, e.g. Suh, J. Y. et al., “Plasmonic Bowtie Nanolaser Arrays,” Nano Lett. 12, 5769-5774 (2012); Zhou, W. et al., “Lasing action in strongly coupled plasmonic nanocavity arrays,” Nature Nanotech. 8, 506-511 (2013). A drawback is that plasmonic sources are typically of low Q as a result of dissipative losses. Nevertheless, enhancements of orders of magnitude in the local electric field strength are available with plasmonic nanostructures in the form of (for example) bowties, spheres, cylinders, or cones, an attribute that is responsible for the detection of single nanoparticles and molecules by Raman scattering. Nie, S. & Emery, S., “Probing single molecules and single nanoparticles by surface-enhanced Raman scattering. Science 275, 1102-1106 (1997).
Some approaches have combined the resonator and gain medium. See, e.g., Sandoghdar, V. et al., “Very low threshold whispering-gallery-mode microsphere laser,” Phys. Rev. A 54, R1777-R1780 (1996); Kuwata-Gonokami, M., Takeda, K., Yasuda, H. & Ema, K. Laser-emission from dye-doped polystyrene microsphere. Japanese J. Appl. Phys. Part 2-Lett. 31, L99-L101 (1992); Cai, M., Painter, O., Vahala, K. J. & Sercel, P. C., “Fiber-coupled microsphere laser,” Opt. Lett. 25, 1430-1432 (2000). Combining the resonator and gain medium is advantageous from the perspective of minimizing the overall volume of the emitter. However, integrating the gain medium and resonator precludes the opportunity to optimize separately the performance of either element. This is particularly true for crystalline microresonators for which controllable doping of the resonator material with the lasant species is problematic. See, e.g., Hartnett, J. G., Locke, C. R., Ivanov, E. N., Tobar, M. E. & Stanwix, P. L., “Cryogenic sapphire oscillator with exceptionally high long-term frequency stability,” Appl. Phys. Lett. 89, 203513 (2006) “Grudinin, I. S., Matsko, A. B. & Maleki, L., “Brillouin lasing with a CaF2 whispering gallery mode resonator,” Phys. Rev. Lett. 102, 043902 (2009).
In virtually all existing micro- and nano-scale optical amplifiers and lasers, the optical intensity builds up from the spontaneous emission background, also known as “the noise”, but doing so limits the temporal coherence of the output radiation.
Injection seeding is a concept that has been used in various devices to introduce weak optical fields into gain media, such as injecting a laser signal into a bulk dye, for the purpose of overcoming the noise quickly, and, thereby, building the optical field intensity more rapidly. See, Farkas, A. M. & Eden, J. G., “Pulsed dye amplification and frequency-doubling of single longitudinal mode semiconductor-lasers,” IEEE J. Quant. Electron. 29, 2923-2927 (1993).
Despite these efforts and advances, the functionality of existing hybrid optoplasmonic systems has been limited. As an example, the existing systems have failed to provide a narrow linewidth emitter integrated with a broadband amplifier. In prior efforts, the amplifier Q had been constrained by the whispering gallery mode resonator. See, Choi, Y. et al., “Ultrahigh-Q microsphere dye laser based on evanescent-wave coupling. J. Kor. Phys. Soc. 39, 928-931 (2001); Kuwata-Gonokami, M. & Takeda, K. Polymer whispering gallery mode lasers. Opt. Mater. 9, 12-17 (1998).